Does the customer expect an increased frequency when the amplitude goes down? You have to always look at the maximum instantaneous flow required and make sure you can satisfy that through the pump (and if installed, the accumulator).
If the maximum instantaneous flow required from the pump/accumulator is less for the low amplitude tests, then the speed of the motor can be reduced.
Once you reduce the motor speed, you have to start looking at whether or not the fan on the motor can keep it cool at the slower speed. Sometimes a separate cooling fan on the motor which is run at constant speed is required.
In general with a VFD you are in a constant torque mode when operating below base frequency so theoretially you should be ok with pressure (pressure is directly proportional to torque). But you should look at the mechanical efficiency of the pump at the reduced speed to make sure it doesn’t drop off dramatically and keep you from making the required pressure. Most pump data sheets give information at standard speeds like 1500 and 1800 RPM. You may have to ask the pump maufacturer to get the efficiency numbers at different rotation speeds.
If you have multiple stage valves, be sure to account for pilot oil flow and pressure reuqirements as well.
If the conditions of the oil supplying the servo valves varries by any significant margins you will likely have to have different tuning parameters for the different conditions. This is pretty easy with the RMC and we use this quite often on a system we do where we vary the system pressure based on tonnage requred on a press for different parts.
Like Jacob said, the energy across the valve is not something that is easy to eliminate. I have seen designs with a servo motor, driving a fixed pump plumbed directly to a cylinder but I don’t think it would be easy at your frequencies. It definitely would be very expensive as you would need a servo motor, drive and pump for each cylinder!